Functionalized Metallic 2D Transition Metal Dichalcogenide-Based Solid-State Electrolyte for Flexible All-Solid-State Supercapacitors

Highly efficient and durable flexible solid-state supercapacitors (FSSSCs) are emerging as low-cost devices for portable and wearable electronics due to the elimination of leakage of toxic/corrosive liquid electrolytes and their capability to withstand elevated mechanical stresses. Nevertheless, the spread of FSSSCs requires the development of durable and highly conductive solid-state electrolytes, whose electrochemical characteristics must be competitive with those of traditional liquid electrolytes. Here, we propose an innovative composite solid-state electrolyte prepared by incorporating metallic two-dimensional group-5 transition metal dichalcogenides, namely, liquid-phase exfoliated functionalized niobium disulfide (f-NbS2) nanoflakes, into a sulfonated poly(ether ether ketone) (SPEEK) polymeric matrix. The terminal sulfonate groups in f-NbS2 nanoflakes interact with the sulfonic acid groups of SPEEK by forming a robust hydrogen bonding network. Consequently, the composite solid-state electrolyte is mechanically/dimensionally stable even at a degree of sulfonation of SPEEK as high as 70.2%. At this degree of sulfonation, the mechanical strength is 38.3 MPa, and thanks to an efficient proton transport through the Grotthuss mechanism, the proton conductivity is as high as 94.4 mS cm–1 at room temperature. To elucidate the importance of the interaction between the electrode materials (including active materials and binders) and the solid-state electrolyte, solid-state supercapacitors were produced using SPEEK and poly(vinylidene fluoride) as proton conducting and nonconducting binders, respectively. The use of our solid-state electrolyte in combination with proton-conducting SPEEK binder and carbonaceous electrode materials (mixture of activated carbon, single/few-layer graphene, and carbon black) results in a solid-state supercapacitor with a specific capacitance of 116 F g–1 at 0.02 A g–1, optimal rate capability (76 F g–1 at 10 A g–1), and electrochemical stability during galvanostatic charge/discharge cycling and folding/bending stresses.

* E-mail: francesco.bonaccorso@iit.it, s.bellani@bedimensional.it solution comprising SPEEK and NMP (1:10 wt/vol) was stirred at 60 ˚C to dissolve the polymer and obtain a homogeneous solution. Afterward, various weight percentages of f-NbS 2 nanoflakes dispersed in NMP were added to the SPEEK solution, keeping the mixture stirred for 4 h at 60 ˚C.
Then, the nanocomposite electrolytes were produced by casting method using a Dr. blade. The asprepared films were dried at 80 ˚C overnight, at 120 ˚C for 12 h, and 140 ˚C for 4 h. Also, the pristine SPEEK electrolyte was prepared in a similar way to the nanocomposite ones but without adding the f-NbS 2 nanoflakes. After drying, the solid-state electrolyte thickness was 80 µm, as measured through contact profilometry (XP2, Ambios Technology). The dried electrolytes were activated with 1 M H 2 SO 4 and then washed with ultrapure water. Solid-state electrolytes based on f-MoS 2 nanoflakes instead of f-NbS 2 ones were also produced for comparison, following the same fabrication procedures. The resulting solid-state electrolytes were used in form of self-standing membranes to fabricate solid-state supercapacitors. Rigid configurations were produced by sandwiching the solid-state electrolytes between two electrodes using a Swagelok cell. Table S1 lists all the investigated devices. Noteworthy, a traditional electrochemical double layer capacitor (EDLC) using PVDF as the binder, 1 M H 2 SO 4 as the liquid (aqueous) electrolyte, and a glass fiber as separator (Whatman) (device named PVDF-1 M H 2 SO 4 ) was also produced and * E-mail: francesco.bonaccorso@iit.it, s.bellani@bedimensional.it characterized as aqueous EDLC for comparison. After the characterization of these rigid supercapacitor configurations, a flexible solid-state supercapacitor (FSSSC) based on the most performant combination of solid-state electrolyte and electrode binder was produced by sandwiching the solid-state electrolyte between two electrodes and copper tape and applying a pressure of 1 MPa. The resulting device was packed using a 0.06 mm-thick Kapton tape. dried under vacuum at room temperature. The morphological and statistical analyses of the lateral dimension of the produced nanoflakes were carried out using ImageJ software (NIH) and OriginPro 9.1 software (OriginLab), respectively. Atomic force microscopy (AFM) images of the exfoliated materials were acquired using a Bruker Dimension Icon atomic force microscope (Bruker Dimension Icon, Billerica, MA, USA). The measurements were carried out in intermittent contact mode using RTESPA cantilevers (Bruker, Billerica, MA, USA) with a tip with a nominal diameter of 8 nm. A driving frequency of ~300 kHz was used for the image acquisition. The images were collected over an area of 2.5×2.5 µm 2 (512×512 data points), using a scan rate of 0.7 Hz. The samples were prepared by depositing the diluted dispersions of the exfoliated materials onto mica substrates (G250-1, Agar Scientific Ltd.). The height profile analysis was performed using Gwydion 2.54 software. The statistical analysis of the AFM data was performed using OriginPro 9.1 software. X-ray diffraction (XRD) analysis of the exfoliated materials was performed using a PANalytical Empyrean X-ray diffractometer with Cu Kα * E-mail: francesco.bonaccorso@iit.it, s.bellani@bedimensional.it radiation, while a Renishaw micro-Raman Invia 1000 spectrophotometer was used for the Raman spectroscopy analysis at 532 nm exciting wavelength. For both XRD and Raman analyses, the samples were prepared by drop casting diluted dispersion of the exfoliated materials onto the Si/SiO 2 substrates and dried under vacuum for 12 h. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) analyses were carried out using a scanning electron microscope (SEM, JEOL JSM-6490LA SEM Analytical microscope).
Fourier-transform infrared (FTIR) spectroscopy measurements of the produced nanocomposite solid-state electrolytes were carried out using an Equinox 70 FTIR spectrometer coupled with the attenuated total reflection (ATR) accessory (MIRacle ATR, Pike Technologies), scanning wavenumbers from 600 to 3800 cm -1 at room temperature.
The water uptake (WU) and membrane swelling (MS) of the solid-state electrolytes were calculated using the following method. First, a cut piece of the produced electrolyte was dried overnight at 90 ˚C, and then its weight (W d ) and area (A d ) were measured. Afterward, the solidstate electrolyte was immersed in deionized water for 12 h at room temperature and after blotdrying the residual water with two clean filter papers, the weight (W w ) and area (A w ) of the * E-mail: francesco.bonaccorso@iit.it, s.bellani@bedimensional.it solid-state electrolyte were immediately measured. Finally, the WU and MS of the solid-state electrolyte were calculated using the following equations: The proton conductivity (σ) of the solid-state electrolytes was measured through electrochemical impedance spectroscopy (EIS) measurements using two platinum electrodes over a frequency range of 2 mHz-100 kHz with an AC voltage amplitude of 0.05 V. The samples were prepared by soaking the as-produced solid-state electrolytes in deionized water for 24 h. The σ of the produced electrolytes was determined by: where L is the electrolyte thickness (cm), A is the electrolyte area (cm 2 ) and R is defined as the electrolyte resistance (Ω) obtained from the Nyquist plots (i.e., real part of the impedance at high frequency > 10 kHz). The tensile tests were performed using a dual column Instron 3365 universal testing machine equipped with a 10 N cell load and pneumatic clamps. Dumbbellshaped ISO 527-2 type 5A samples with a thickness of 1 mm were cut out directly from prepared * E-mail: francesco.bonaccorso@iit.it, s.bellani@bedimensional.it solid-state electrolytes. Each sample was measured three times, averaging the extrapolated characteristics. Strain displacement was applied with rates ranging 0.2 mm min -1 at 25 ˚C.
Thermogravimetric analysis (TGA) was performed to investigate the thermal stability of the produced solid-state electrolytes using a TGA Q500 (TA Instruments, USA) thermogravimetric analyzer in N 2 atmosphere from 25 to 800 ˚C at a heating rate of 10 ˚C min -1 . The electrochemical characterization of the supercapacitors included cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and EIS measurements, which were performed using a potentiostat/galvanostat (VMP3, Biologic) at room temperature (25 °C). Cyclic voltammetry measurements were carried out at voltage scan rates ranging from 5 mV s -1 to 1500 V s -1 .
Galvanostatic charge/discharge measurements were carried out at different specific currents, ranging from 0.02 to 50 A g -1 . The specific (gravimetric) capacitance (C g ) of the electrode was calculated according to the following equations: from CV curves (4) C g = ∫idV ms∆V from GCD curves (5) where ΔV is the voltage cell window, is the integrated area of the CV curve, m is the mass ∫idV loading of the electrode (excluding the current collector), s is the scan rate, V is the cell voltage, i is the charging/discharging current, t d is the discharge time of the GCD curve.
The energy density and power density of the supercapacitors were calculated by: energy density (Wh kg -1 ) = (6)        Figure S6. Electrochemical characterization of the FSSSC. (a) Specific capacitance vs. specific current plot measured for the SPEEK-2.5%-f-NbS 2 :SPEEK-based FSSSC in normal state. (b) CV curves measured for the FSSSC folded at 180 ⁰ and (c) after 1000 bending cycles at a curvature radius of 2 cm, at voltage scan rates ranging from 40 to 1500 mV s -1 .